Users began reporting that outage just before 8 a.m. ET, according to Downdetector.
It appears most people are reporting problems when trying to use the website version.
Microsoft 365 posted on social media that they are investigating the issue and that a fix has been deployed.
Services were slowly being restored as of 12:30 p.m. ET.
Downdetector is warning using to beware of scammers who may attempt to offer fake support for Outlook. If you have paid for support, they recommend contacting your financial institution to review your account's security and to report any fraud.